1. Field of the Invention
This invention relates to a method for the recovery of Zn and Pb through volatilization from iron and steel dust.
2. Description of Prior Art
The iron and steel dust which occurs from an open-arc furnace and a blast furnace operated in the iron and steel industry contains Zn and Pb, as well as iron oxide. The industry has made an established rule to recover such Zn and Pb from the iron and steel dust. Heretofore, this recovery has been most popularly effected by the reducing roasting process represented by the Waelz kiln process, involving the use of an internally heated counterflow type rotary kiln. The reducing roasting method using this particular rotary kiln comprises roasting the iron and steel dust in a strongly reducing atmosphere under suitably selected conditions of temperature and retention time, thereby separating Zn and Pb through volatilization from the dust and enabling iron to be discharged in the form of ordinary solid, reduced iron pellets. However, it is extremely difficult to maintain the operation of this rotary kiln under appropriate conditions for a long time. In terms of the recovery of Zn and Pb through volatilization, this process is not amply satisfactory. Intensification of the reducing atmosphere and elevation of the roasting temperature are effective in heightening the ratio of volatilization of Zn, etc. If the reduction is carried out at a higher temperature, the component of low melting point is deposited in a fused state on the inner wall surface of the rotary kiln, giving rise to ring-shaped deposits and, consequently, impeding the continued operation of the rotary kiln. Since the elevation of temperature thus results in a decrease of the duration of practicable continued operation, there is a limit to the temperature elevation. As the result, the ratio of volatilization of Zn, etc., is inevitably reduced to some extent. As a measure to cope with this difficulty, the method which comprises adding a flux to the feed to the rotary kiln, thereby adjusting the melting point of the feed and enabling the feed to be completely melted within the rotary kiln, has found wide acceptance. By this method, the ratio of volatilization of Zn and Pb is satisfactorily high and the formation of deposits on the wall of the kiln is avoided in the region in which the feed is completely melted. This method, however, has the disadvantage that in the half-melted region in which the feed is in the process of being transformed from its solid state to its fused state, formation of deposits on the kiln wall occurs conspicuously and, consequently, the duration of continued kiln operation is extremely short.
The recovery of Zn and Pb from the iron and steel dust is otherwise effected by the slag fuming method which, by the use of a stationary furnace in the place of the rotary kiln, effects the desired volatilization of Zn, etc., by melting the iron and steel dust completely, as by electric heating and blowing air and a reducing agent such as coal or coke through a lance pipe into the molten iron and steel dust. This method has the disadvantage that, since the reduction, when carried out too strongly, produces metallic iron and offers obstacles to the furnace operation, the degree of the reduction is repressed to the extent of avoiding the formation of metallic iron and, consequently, the ratio of volatilization of Zn, etc., cannot be increased sufficiently.